BUG:Resample with groupby & agg()

[BassKot]

• [ X] I have checked that this issue has not already been reported.

• [ all versions] I have confirmed this bug exists on the latest version of pandas.

• (optional) I have confirmed this bug exists on the master branch of pandas.

---

Note: Please read this guide detailing how to provide the necessary information for us to reproduce your bug.

Code Sample, a copy-pastable example

import pandas as pd
import numpy as np

data = pd.DataFrame({
    'cat': ['cat_1', 'cat_1', 'cat_2', 'cat_1', 'cat_2', 'cat_1', 'cat_2', 'cat_1'],
    'num': [5,20,22,3,4,30,10,50],
    'date': ['2019-2-1', '2018-02-03','2020-3-11','2019-2-2', '2019-2-2', '2018-12-4','2020-3-11', '2020-12-12']
})
data['date'] = pd.to_datetime(data['date'])
aggreg = data.groupby('cat').resample('Y', on='date')
summ_ = aggreg.sum()
agg_summ_ = aggreg.agg({'num': 'sum'})
summ_
agg_summ_

Problem description

When I want aggregate all columns by sum, example with summ_ calculates normal, but if I do example with agg it calculates incorrect.

Expected Output

summ_

Output of pd.show_versions()

INSTALLED VERSIONS ------------------ commit : d9fff27 python : 3.7.4.final.0 python-bits : 64 OS : Darwin OS-release : 19.6.0 Version : Darwin Kernel Version 19.6.0: Sun Jul 5 00:43:10 PDT 2020; root:xnu-6153.141.1~9/RELEASE_X86_64 machine : x86_64 processor : i386 byteorder : little LC_ALL : en_US.UTF-8 LANG : ru_RU.UTF-8 LOCALE : en_US.UTF-8

pandas : 1.1.0
numpy : 1.18.4
pytz : 2019.3
dateutil : 2.8.0
pip : 20.0.2
setuptools : 41.4.0
Cython : 0.29.13
pytest : 5.2.1
hypothesis : None
sphinx : 2.2.0
blosc : None
feather : None
xlsxwriter : 1.2.1
lxml.etree : 4.4.1
html5lib : 1.0.1
pymysql : 0.9.3
psycopg2 : None
jinja2 : 2.11.2
IPython : 7.8.0
pandas_datareader: None
bs4 : 4.8.0
bottleneck : 1.2.1
fsspec : 0.5.2
fastparquet : None
gcsfs : None
matplotlib : 3.2.1
numexpr : 2.7.0
odfpy : None
openpyxl : 3.0.0
pandas_gbq : None
pyarrow : None
pytables : None
pyxlsb : None
s3fs : None
scipy : 1.4.1
sqlalchemy : 1.3.9
tables : 3.5.2
tabulate : 0.8.7
xarray : 0.16.0
xlrd : 1.2.0
xlwt : 1.3.0
numba : 0.45.1

[jreback]

pls show an actually reproducible example ; construct the input with code and show the results and what is not correct

[BassKot]

pls show an actually reproducible example ; construct the input with code and show the results and what is not correct

Done

[Liam3851]

Looks like a dupe of #33548.

[simonjayhawkins]

Looks like a dupe of #33548.

closing as duplicate